Highly filled granular pastes, comprising metal particles (0.1-100 microns) at a volume percent greater than 65% in an organic binder, are important in a variety of industries including the food, oral care, and microelectronics industries.
In microelectronics these granular pastes often referred to as thermal pastes, play an important role in packaging technology by facilitating heat removal from the chip and accommodating mechanical stresses induced by thermal expansion mismatches between the silicon die and spreader/hat. The proper dispensing of thermal pastes, such that the entire chip area is adequately and homogeneously covered, is a major concern that has been addressed through the formulation of thermal paste preforms. Thermal paste preforms are described in U.S. Pat. No. 6,444,496, which is incorporated herein by reference. These preforms are of size approximately equal to the chip area and are applied to the backside of the chip and compressed between the chip and heat spreader or heat sink to the final desired thickness.
For optimal transfer of these preforms to the backside of the chip, the preforms are prepared on a thin backing material that is 5 mils or less with a typical thickness between about 1 and about 2 mils that facilitates paste peel off with minimal adhesion and release with minimal paste retains. Retained material can result in minor changes in the composition of the preform applied to the die with potentially major changes in its properties such as viscosity and thermal conductivity which are highly sensitive to solids loading for these highly filled granular pastes. The highly filled granular pastes typically have a solids volume fraction greater than about 65%. The retained material can also introduce microscopic roughness in the preform that could serve as sites for air entrapment with subsequent negative impact on paste thermal conductivity and reliability. The ideal properties of the preform backing material therefore include low surface energy, negligible surface roughness, and relatively good thermal stability. Fluoropolymers and polyesters are common polymer films used for release applications. Fluoropolymer films in general have better release properties than polyesters, have good thermal stability, but are more expensive and can have surface roughness on the order of 200 nanometers.